1. Field.
This invention relates to improvements in the production of metal articles having a multiple-layer protective coating comprising successive layers of zinc, chromium and synthetic resin.
2. Description of the Prior Art.
During the past the requirements of the automobile industry regarding the corrosion resistance of the parts which are vital for the safety of the vehicle, as for instance brake conduits, have become more stringent and as a consequence correspondingly stricter regulations have been enacted regarding their quality that affect all production stages of such parts, thus giving rise to the problem of the present invention of providing a coating for corrosion protection which meets the requirements for the finished article and which is not only adequate to tolerate the mechanical deformation to which the corrosion protected article is subjected during manufacture and finishing operations, but which is also particularly suited to tolerate such treatment. Finally, it has to be kept in mind that the automotive industry has enormous quantity demands, as in the case of brake conduits for automobiles, so that a method had to be found which in practice has a very high production capacity.
A practical solution to the above problems is the method described in our co-pending U.S. patent application, Ser. No. 179,524, now U.S. Pat. No. 3808057, which generally employs the following sequence of steps:
A. Zinc plating is developed on a metal base body as a highly homogenous, fine crystalline structure. This may be accomplished by precipitation of the zinc plating in a sulfuric acid electrolyte at extremely high current densities of between 40 to 100 A/dm.sup.2, the electrolyte temperature ranging between 50.degree. and 55.degree. C., and the electrolyte being vigorously agitated and including organic compounds.
B. A chromate plating on the zinc plated base body is produced in a chromic acid solution with a pH value of less than 2, and the chromated base body is rinsed in order to remove adhering residues of chromic acid solution.
C. The wet chromated base body is coated with a synthetic resin dispersion, and the coated base body is subjected to thermal treatment to convert the chromate coating to trivalent chromium and enable the synthetic resin to permeate the chromate coating.
The above is a summary of the method of our U.S. patent, and is qualified by reference to the patent itself. Our present invention contemplates still further improvements which are to be achieved by modification of certain features of the invention of our aforesaid U.S. patent.